build: use MDZ_DEPS env var to determine dependencies type

[zwpaper]

this is how I propose the new solution for local dependencies.

As before, I believe local dependencies should not be in-tree for Go.

this PR added and env variable MDZ_DEPS which could be local, and could be used by

export MDZ_DEPS=local
make

or

MDZ_DEPS=local make

which will change the agent dependency to local ../agent when building, and we can still keep the versioned dependency in-tree.

for local development, we can export MDZ_DEPS in shell, and it will use local dependencies automatically every time we build in this shell.

[zwpaper]

cc @gaocegege @kemingy

[gaocegege]

How to support the goreleaser with this PR? https://github.com/tensorchord/openmodelz/blob/main/.goreleaser.yaml

[zwpaper]

after the discussion and getting familiar with openmodelz, I was really curious about why prefer separate openmodelz into 4 go packages rather than a single one, if the 4 packages are so closely related...

let's finish the discussion #147 first